They aren't, except for Lukaku. Don't think he was talking about that though.
Any player under 21 doesn't have to be registered in the 25 man squad.
We are allowed a maximum of 17 players that don't fit either homegrown criteria (club-trained or not).
We currently have 10, so there is plenty of scope, especially if we pick up a homegrown back up keeper (like Ruddy - not that I'd be fond of spending 10m on him, though).